diff --git a/CHANGELOG_PENDING.md b/CHANGELOG_PENDING.md
index 50d60f62b..3f8eb17f9 100644
--- a/CHANGELOG_PENDING.md
+++ b/CHANGELOG_PENDING.md
@@ -18,6 +18,7 @@ Friendly reminder, we have a [bug bounty program](https://hackerone.com/tendermi
 - Go API
 
   - [privval] [\#4744](https://github.com/tendermint/tendermint/pull/4744) Remove deprecated `OldFilePV` (@melekes)
+  - [mempool] [\#4759](https://github.com/tendermint/tendermint/pull/4759) Modify `Mempool#InitWAL` to return an error (@melekes)
 
 - Blockchain Protocol
 
@@ -32,6 +33,7 @@ Friendly reminder, we have a [bug bounty program](https://hackerone.com/tendermi
   - nostrip: don't strip debugging symbols nor DWARF tables.
   - cleveldb: use cleveldb as db backend instead of goleveldb.
   - race: pass -race to go build and enable data race detection.
+- [mempool] [\#4759](https://github.com/tendermint/tendermint/pull/4759) Allow ReapX and CheckTx functions to run in parallel (@melekes)
 
 ### BUG FIXES:
 
diff --git a/libs/os/os.go b/libs/os/os.go
index b56726c94..4773feef0 100644
--- a/libs/os/os.go
+++ b/libs/os/os.go
@@ -46,7 +46,7 @@ func EnsureDir(dir string, mode os.FileMode) error {
 	if _, err := os.Stat(dir); os.IsNotExist(err) {
 		err := os.MkdirAll(dir, mode)
 		if err != nil {
-			return fmt.Errorf("could not create directory %v. %v", dir, err)
+			return fmt.Errorf("could not create directory %v: %w", dir, err)
 		}
 	}
 	return nil
diff --git a/mempool/clist_mempool.go b/mempool/clist_mempool.go
index 7732032a4..f55d62329 100644
--- a/mempool/clist_mempool.go
+++ b/mempool/clist_mempool.go
@@ -7,9 +7,6 @@ import (
 	"fmt"
 	"sync"
 	"sync/atomic"
-	"time"
-
-	"github.com/pkg/errors"
 
 	abci "github.com/tendermint/tendermint/abci/types"
 	cfg "github.com/tendermint/tendermint/config"
@@ -32,9 +29,8 @@ import (
 // be efficiently accessed by multiple concurrent readers.
 type CListMempool struct {
 	// Atomic integers
-	height     int64 // the last block Update()'d to
-	txsBytes   int64 // total size of mempool, in bytes
-	rechecking int32 // for re-checking filtered txs on Update()
+	height   int64 // the last block Update()'d to
+	txsBytes int64 // total size of mempool, in bytes
 
 	// notify listeners (ie. consensus) when txs are available
 	notifiedTxsAvailable bool
@@ -42,15 +38,19 @@ type CListMempool struct {
 
 	config *cfg.MempoolConfig
 
-	proxyMtx     sync.Mutex
+	// Exclusive mutex for Update method to prevent concurrent execution of
+	// CheckTx or ReapMaxBytesMaxGas(ReapMaxTxs) methods.
+	updateMtx sync.RWMutex
+	preCheck  PreCheckFunc
+	postCheck PostCheckFunc
+
+	wal          *auto.AutoFile // a log of mempool txs
+	txs          *clist.CList   // concurrent linked-list of good txs
 	proxyAppConn proxy.AppConnMempool
-	txs          *clist.CList // concurrent linked-list of good txs
-	preCheck     PreCheckFunc
-	postCheck    PostCheckFunc
 
 	// Track whether we're rechecking txs.
-	// These are not protected by a mutex and are expected to be mutated
-	// in serial (ie. by abci responses which are called in serial).
+	// These are not protected by a mutex and are expected to be mutated in
+	// serial (ie. by abci responses which are called in serial).
 	recheckCursor *clist.CElement // next expected response
 	recheckEnd    *clist.CElement // re-checking stops here
 
@@ -62,9 +62,6 @@ type CListMempool struct {
 	// This reduces the pressure on the proxyApp.
 	cache txCache
 
-	// A log of mempool txs
-	wal *auto.AutoFile
-
 	logger log.Logger
 
 	metrics *Metrics
@@ -87,7 +84,6 @@ func NewCListMempool(
 		proxyAppConn:  proxyAppConn,
 		txs:           clist.New(),
 		height:        height,
-		rechecking:    0,
 		recheckCursor: nil,
 		recheckEnd:    nil,
 		logger:        log.NewNopLogger(),
@@ -132,55 +128,64 @@ func WithMetrics(metrics *Metrics) CListMempoolOption {
 	return func(mem *CListMempool) { mem.metrics = metrics }
 }
 
-// *panics* if can't create directory or open file.
-// *not thread safe*
-func (mem *CListMempool) InitWAL() {
-	walDir := mem.config.WalDir()
-	err := tmos.EnsureDir(walDir, 0700)
-	if err != nil {
-		panic(errors.Wrap(err, "Error ensuring WAL dir"))
+func (mem *CListMempool) InitWAL() error {
+	var (
+		walDir  = mem.config.WalDir()
+		walFile = walDir + "/wal"
+	)
+
+	const perm = 0700
+	if err := tmos.EnsureDir(walDir, perm); err != nil {
+		return err
 	}
-	af, err := auto.OpenAutoFile(walDir + "/wal")
+
+	af, err := auto.OpenAutoFile(walFile)
 	if err != nil {
-		panic(errors.Wrap(err, "Error opening WAL file"))
+		return fmt.Errorf("can't open autofile %s: %w", walFile, err)
 	}
+
 	mem.wal = af
+	return nil
 }
 
 func (mem *CListMempool) CloseWAL() {
-	mem.proxyMtx.Lock()
-	defer mem.proxyMtx.Unlock()
-
 	if err := mem.wal.Close(); err != nil {
 		mem.logger.Error("Error closing WAL", "err", err)
 	}
 	mem.wal = nil
 }
 
+// Safe for concurrent use by multiple goroutines.
 func (mem *CListMempool) Lock() {
-	mem.proxyMtx.Lock()
+	mem.updateMtx.Lock()
 }
 
+// Safe for concurrent use by multiple goroutines.
 func (mem *CListMempool) Unlock() {
-	mem.proxyMtx.Unlock()
+	mem.updateMtx.Unlock()
 }
 
+// Safe for concurrent use by multiple goroutines.
 func (mem *CListMempool) Size() int {
 	return mem.txs.Len()
 }
 
+// Safe for concurrent use by multiple goroutines.
 func (mem *CListMempool) TxsBytes() int64 {
 	return atomic.LoadInt64(&mem.txsBytes)
 }
 
+// Lock() must be help by the caller during execution.
 func (mem *CListMempool) FlushAppConn() error {
 	return mem.proxyAppConn.FlushSync()
 }
 
+// XXX: Unsafe! Calling Flush may leave mempool in inconsistent state.
 func (mem *CListMempool) Flush() {
-	mem.proxyMtx.Lock()
-	defer mem.proxyMtx.Unlock()
+	mem.updateMtx.RLock()
+	defer mem.updateMtx.RUnlock()
 
+	_ = atomic.SwapInt64(&mem.txsBytes, 0)
 	mem.cache.Reset()
 
 	for e := mem.txs.Front(); e != nil; e = e.Next() {
@@ -188,13 +193,17 @@ func (mem *CListMempool) Flush() {
 		e.DetachPrev()
 	}
 
-	mem.txsMap = sync.Map{}
-	_ = atomic.SwapInt64(&mem.txsBytes, 0)
+	mem.txsMap.Range(func(key, _ interface{}) bool {
+		mem.txsMap.Delete(key)
+		return true
+	})
 }
 
 // TxsFront returns the first transaction in the ordered list for peer
 // goroutines to call .NextWait() on.
 // FIXME: leaking implementation details!
+//
+// Safe for concurrent use by multiple goroutines.
 func (mem *CListMempool) TxsFront() *clist.CElement {
 	return mem.txs.Front()
 }
@@ -202,6 +211,8 @@ func (mem *CListMempool) TxsFront() *clist.CElement {
 // TxsWaitChan returns a channel to wait on transactions. It will be closed
 // once the mempool is not empty (ie. the internal `mem.txs` has at least one
 // element)
+//
+// Safe for concurrent use by multiple goroutines.
 func (mem *CListMempool) TxsWaitChan() <-chan struct{} {
 	return mem.txs.WaitChan()
 }
@@ -210,21 +221,17 @@ func (mem *CListMempool) TxsWaitChan() <-chan struct{} {
 // cb: A callback from the CheckTx command.
 //     It gets called from another goroutine.
 // CONTRACT: Either cb will get called, or err returned.
-func (mem *CListMempool) CheckTx(tx types.Tx, cb func(*abci.Response), txInfo TxInfo) (err error) {
-	mem.proxyMtx.Lock()
+//
+// Safe for concurrent use by multiple goroutines.
+func (mem *CListMempool) CheckTx(tx types.Tx, cb func(*abci.Response), txInfo TxInfo) error {
+	mem.updateMtx.RLock()
 	// use defer to unlock mutex because application (*local client*) might panic
-	defer mem.proxyMtx.Unlock()
+	defer mem.updateMtx.RUnlock()
 
-	var (
-		memSize  = mem.Size()
-		txsBytes = mem.TxsBytes()
-		txSize   = len(tx)
-	)
-	if memSize >= mem.config.Size ||
-		int64(txSize)+txsBytes > mem.config.MaxTxsBytes {
-		return ErrMempoolIsFull{
-			memSize, mem.config.Size,
-			txsBytes, mem.config.MaxTxsBytes}
+	txSize := len(tx)
+
+	if err := mem.isFull(txSize); err != nil {
+		return err
 	}
 
 	// The size of the corresponding amino-encoded TxMessage
@@ -274,7 +281,7 @@ func (mem *CListMempool) CheckTx(tx types.Tx, cb func(*abci.Response), txInfo Tx
 	// END WAL
 
 	// NOTE: proxyAppConn may error if tx buffer is full
-	if err = mem.proxyAppConn.Error(); err != nil {
+	if err := mem.proxyAppConn.Error(); err != nil {
 		return err
 	}
 
@@ -290,7 +297,9 @@ func (mem *CListMempool) CheckTx(tx types.Tx, cb func(*abci.Response), txInfo Tx
 // and peerID is not included in the ABCI request, so we have to set request-specific callbacks that
 // include this information. If we're not in the midst of a recheck, this function will just return,
 // so the request specific callback can do the work.
-// When rechecking, we don't need the peerID, so the recheck callback happens here.
+//
+// When rechecking, we don't need the peerID, so the recheck callback happens
+// here.
 func (mem *CListMempool) globalCb(req *abci.Request, res *abci.Response) {
 	if mem.recheckCursor == nil {
 		return
@@ -359,6 +368,22 @@ func (mem *CListMempool) removeTx(tx types.Tx, elem *clist.CElement, removeFromC
 	}
 }
 
+func (mem *CListMempool) isFull(txSize int) error {
+	var (
+		memSize  = mem.Size()
+		txsBytes = mem.TxsBytes()
+	)
+
+	if memSize >= mem.config.Size || int64(txSize)+txsBytes > mem.config.MaxTxsBytes {
+		return ErrMempoolIsFull{
+			memSize, mem.config.Size,
+			txsBytes, mem.config.MaxTxsBytes,
+		}
+	}
+
+	return nil
+}
+
 // callback, which is called after the app checked the tx for the first time.
 //
 // The case where the app checks the tx for the second and subsequent times is
@@ -376,6 +401,15 @@ func (mem *CListMempool) resCbFirstTime(
 			postCheckErr = mem.postCheck(tx, r.CheckTx)
 		}
 		if (r.CheckTx.Code == abci.CodeTypeOK) && postCheckErr == nil {
+			// Check mempool isn't full again to reduce the chance of exceeding the
+			// limits.
+			if err := mem.isFull(len(tx)); err != nil {
+				// remove from cache (mempool might have a space later)
+				mem.cache.Remove(tx)
+				mem.logger.Error(err.Error())
+				return
+			}
+
 			memTx := &mempoolTx{
 				height:    mem.height,
 				gasWanted: r.CheckTx.GasWanted,
@@ -437,7 +471,6 @@ func (mem *CListMempool) resCbRecheck(req *abci.Request, res *abci.Response) {
 		}
 		if mem.recheckCursor == nil {
 			// Done!
-			atomic.StoreInt32(&mem.rechecking, 0)
 			mem.logger.Info("Done rechecking txs")
 
 			// incase the recheck removed all txs
@@ -450,6 +483,7 @@ func (mem *CListMempool) resCbRecheck(req *abci.Request, res *abci.Response) {
 	}
 }
 
+// Safe for concurrent use by multiple goroutines.
 func (mem *CListMempool) TxsAvailable() <-chan struct{} {
 	return mem.txsAvailable
 }
@@ -468,17 +502,15 @@ func (mem *CListMempool) notifyTxsAvailable() {
 	}
 }
 
+// Safe for concurrent use by multiple goroutines.
 func (mem *CListMempool) ReapMaxBytesMaxGas(maxBytes, maxGas int64) types.Txs {
-	mem.proxyMtx.Lock()
-	defer mem.proxyMtx.Unlock()
+	mem.updateMtx.RLock()
+	defer mem.updateMtx.RUnlock()
 
-	for atomic.LoadInt32(&mem.rechecking) > 0 {
-		// TODO: Something better?
-		time.Sleep(time.Millisecond * 10)
-	}
-
-	var totalBytes int64
-	var totalGas int64
+	var (
+		totalBytes int64
+		totalGas   int64
+	)
 	// TODO: we will get a performance boost if we have a good estimate of avg
 	// size per tx, and set the initial capacity based off of that.
 	// txs := make([]types.Tx, 0, tmmath.MinInt(mem.txs.Len(), max/mem.avgTxSize))
@@ -505,19 +537,15 @@ func (mem *CListMempool) ReapMaxBytesMaxGas(maxBytes, maxGas int64) types.Txs {
 	return txs
 }
 
+// Safe for concurrent use by multiple goroutines.
 func (mem *CListMempool) ReapMaxTxs(max int) types.Txs {
-	mem.proxyMtx.Lock()
-	defer mem.proxyMtx.Unlock()
+	mem.updateMtx.RLock()
+	defer mem.updateMtx.RUnlock()
 
 	if max < 0 {
 		max = mem.txs.Len()
 	}
 
-	for atomic.LoadInt32(&mem.rechecking) > 0 {
-		// TODO: Something better?
-		time.Sleep(time.Millisecond * 10)
-	}
-
 	txs := make([]types.Tx, 0, tmmath.MinInt(mem.txs.Len(), max))
 	for e := mem.txs.Front(); e != nil && len(txs) <= max; e = e.Next() {
 		memTx := e.Value.(*mempoolTx)
@@ -526,6 +554,7 @@ func (mem *CListMempool) ReapMaxTxs(max int) types.Txs {
 	return txs
 }
 
+// Lock() must be help by the caller during execution.
 func (mem *CListMempool) Update(
 	height int64,
 	txs types.Txs,
@@ -593,7 +622,6 @@ func (mem *CListMempool) recheckTxs() {
 		panic("recheckTxs is called, but the mempool is empty")
 	}
 
-	atomic.StoreInt32(&mem.rechecking, 1)
 	mem.recheckCursor = mem.txs.Front()
 	mem.recheckEnd = mem.txs.Back()
 
diff --git a/mempool/doc.go b/mempool/doc.go
index ddd47aa2d..7e6363e12 100644
--- a/mempool/doc.go
+++ b/mempool/doc.go
@@ -6,19 +6,18 @@
 // safely by calling .NextWait() on each element.
 
 // So we have several go-routines:
-// 1. Consensus calling Update() and Reap() synchronously
+// 1. Consensus calling Update() and ReapMaxBytesMaxGas() synchronously
 // 2. Many mempool reactor's peer routines calling CheckTx()
 // 3. Many mempool reactor's peer routines traversing the txs linked list
-// 4. Another goroutine calling GarbageCollectTxs() periodically
 
 // To manage these goroutines, there are three methods of locking.
 // 1. Mutations to the linked-list is protected by an internal mtx (CList is goroutine-safe)
 // 2. Mutations to the linked-list elements are atomic
-// 3. CheckTx() calls can be paused upon Update() and Reap(), protected by .proxyMtx
+// 3. CheckTx() and/or ReapMaxBytesMaxGas() calls can be paused upon Update(), protected by .updateMtx
 
-// Garbage collection of old elements from mempool.txs is handlde via
-// the DetachPrev() call, which makes old elements not reachable by
-// peer broadcastTxRoutine() automatically garbage collected.
+// Garbage collection of old elements from mempool.txs is handlde via the
+// DetachPrev() call, which makes old elements not reachable by peer
+// broadcastTxRoutine().
 
 // TODO: Better handle abci client errors. (make it automatically handle connection errors)
 package mempool
diff --git a/mempool/mempool.go b/mempool/mempool.go
index 97919ab99..68eec8674 100644
--- a/mempool/mempool.go
+++ b/mempool/mempool.go
@@ -37,7 +37,7 @@ type Mempool interface {
 
 	// Update informs the mempool that the given txs were committed and can be discarded.
 	// NOTE: this should be called *after* block is committed by consensus.
-	// NOTE: unsafe; Lock/Unlock must be managed by caller
+	// NOTE: Lock/Unlock must be managed by caller
 	Update(
 		blockHeight int64,
 		blockTxs types.Txs,
@@ -48,6 +48,7 @@ type Mempool interface {
 
 	// FlushAppConn flushes the mempool connection to ensure async reqResCb calls are
 	// done. E.g. from CheckTx.
+	// NOTE: Lock/Unlock must be managed by caller
 	FlushAppConn() error
 
 	// Flush removes all transactions from the mempool and cache
@@ -68,8 +69,9 @@ type Mempool interface {
 	// TxsBytes returns the total size of all txs in the mempool.
 	TxsBytes() int64
 
-	// InitWAL creates a directory for the WAL file and opens a file itself.
-	InitWAL()
+	// InitWAL creates a directory for the WAL file and opens a file itself. If
+	// there is an error, it will be of type *PathError.
+	InitWAL() error
 
 	// CloseWAL closes and discards the underlying WAL file.
 	// Any further writes will not be relayed to disk.
diff --git a/mock/mempool.go b/mock/mempool.go
index 8c5b6e38f..be690efaa 100644
--- a/mock/mempool.go
+++ b/mock/mempool.go
@@ -38,5 +38,5 @@ func (Mempool) TxsBytes() int64               { return 0 }
 func (Mempool) TxsFront() *clist.CElement    { return nil }
 func (Mempool) TxsWaitChan() <-chan struct{} { return nil }
 
-func (Mempool) InitWAL()  {}
-func (Mempool) CloseWAL() {}
+func (Mempool) InitWAL() error { return nil }
+func (Mempool) CloseWAL()      {}
diff --git a/node/node.go b/node/node.go
index 06125c762..744422d36 100644
--- a/node/node.go
+++ b/node/node.go
@@ -773,7 +773,10 @@ func (n *Node) OnStart() error {
 	n.isListening = true
 
 	if n.config.Mempool.WalEnabled() {
-		n.mempool.InitWAL() // no need to have the mempool wal during tests
+		err = n.mempool.InitWAL()
+		if err != nil {
+			return fmt.Errorf("init mempool WAL: %w", err)
+		}
 	}
 
 	// Start the switch (the P2P server).