From 8c9c1487727a5e7938ca6ee7440334123a467b25 Mon Sep 17 00:00:00 2001 From: Michael Pratt Date: Thu, 14 Oct 2021 18:18:49 -0400 Subject: [release-branch.go1.16] runtime: consistently access pollDesc r/w Gs with atomics Both netpollblock and netpollunblock read gpp using a non-atomic load. When consuming a ready event, netpollblock clears gpp using a non-atomic store, thus skipping a barrier. Thus on systems with weak memory ordering, a sequence like so this is possible: T1 T2 1. netpollblock: read gpp -> pdReady 2. netpollblock: store gpp -> 0 3. netpollunblock: read gpp -> pdReady 4. netpollunblock: return i.e., without a happens-before edge between (2) and (3), netpollunblock may read the stale value of gpp. Switch these access to use atomic loads and stores in order to create these edges. For ease of future maintainance, I've simply changed rg and wg to always be accessed atomically, though I don't believe pollOpen or pollClose require atomics today. For #48925 Fixes #49009 Change-Id: I903ea667eea320277610b4f969129935731520c3 Reviewed-on: https://go-review.googlesource.com/c/go/+/355952 Trust: Michael Pratt Run-TryBot: Michael Pratt TryBot-Result: Go Bot Reviewed-by: Michael Knyszek Reviewed-by: David Chase (cherry picked from commit 1b072b3ed56c18619587354f499fcda5279718a2) Reviewed-on: https://go-review.googlesource.com/c/go/+/356370 --- src/runtime/netpoll.go | 43 ++++++++++++++++++++++++++----------------- 1 file changed, 26 insertions(+), 17 deletions(-) diff --git a/src/runtime/netpoll.go b/src/runtime/netpoll.go index 77eb3aa4c6..f296b0a4db 100644 --- a/src/runtime/netpoll.go +++ b/src/runtime/netpoll.go @@ -74,6 +74,7 @@ type pollDesc struct { // pollReset, pollWait, pollWaitCanceled and runtime·netpollready (IO readiness notification) // proceed w/o taking the lock. So closing, everr, rg, rd, wg and wd are manipulated // in a lock-free way by all operations. + // TODO(golang.org/issue/49008): audit these lock-free fields for continued correctness. // NOTE(dvyukov): the following code uses uintptr to store *g (rg/wg), // that will blow up when GC starts moving objects. lock mutex // protects the following fields @@ -82,11 +83,11 @@ type pollDesc struct { everr bool // marks event scanning error happened user uint32 // user settable cookie rseq uintptr // protects from stale read timers - rg uintptr // pdReady, pdWait, G waiting for read or nil + rg uintptr // pdReady, pdWait, G waiting for read or nil. Accessed atomically. rt timer // read deadline timer (set if rt.f != nil) rd int64 // read deadline wseq uintptr // protects from stale write timers - wg uintptr // pdReady, pdWait, G waiting for write or nil + wg uintptr // pdReady, pdWait, G waiting for write or nil. Accessed atomically. wt timer // write deadline timer wd int64 // write deadline self *pollDesc // storage for indirect interface. See (*pollDesc).makeArg. @@ -143,20 +144,22 @@ func poll_runtime_isPollServerDescriptor(fd uintptr) bool { func poll_runtime_pollOpen(fd uintptr) (*pollDesc, int) { pd := pollcache.alloc() lock(&pd.lock) - if pd.wg != 0 && pd.wg != pdReady { + wg := atomic.Loaduintptr(&pd.wg) + if wg != 0 && wg != pdReady { throw("runtime: blocked write on free polldesc") } - if pd.rg != 0 && pd.rg != pdReady { + rg := atomic.Loaduintptr(&pd.rg) + if rg != 0 && rg != pdReady { throw("runtime: blocked read on free polldesc") } pd.fd = fd pd.closing = false pd.everr = false pd.rseq++ - pd.rg = 0 + atomic.Storeuintptr(&pd.rg, 0) pd.rd = 0 pd.wseq++ - pd.wg = 0 + atomic.Storeuintptr(&pd.wg, 0) pd.wd = 0 pd.self = pd unlock(&pd.lock) @@ -171,10 +174,12 @@ func poll_runtime_pollClose(pd *pollDesc) { if !pd.closing { throw("runtime: close polldesc w/o unblock") } - if pd.wg != 0 && pd.wg != pdReady { + wg := atomic.Loaduintptr(&pd.wg) + if wg != 0 && wg != pdReady { throw("runtime: blocked write on closing polldesc") } - if pd.rg != 0 && pd.rg != pdReady { + rg := atomic.Loaduintptr(&pd.rg) + if rg != 0 && rg != pdReady { throw("runtime: blocked read on closing polldesc") } netpollclose(pd.fd) @@ -198,9 +203,9 @@ func poll_runtime_pollReset(pd *pollDesc, mode int) int { return errcode } if mode == 'r' { - pd.rg = 0 + atomic.Storeuintptr(&pd.rg, 0) } else if mode == 'w' { - pd.wg = 0 + atomic.Storeuintptr(&pd.wg, 0) } return pollNoError } @@ -410,6 +415,8 @@ func netpollgoready(gp *g, traceskip int) { // returns true if IO is ready, or false if timedout or closed // waitio - wait only for completed IO, ignore errors +// Concurrent calls to netpollblock in the same mode are forbidden, as pollDesc +// can hold only a single waiting goroutine for each mode. func netpollblock(pd *pollDesc, mode int32, waitio bool) bool { gpp := &pd.rg if mode == 'w' { @@ -418,17 +425,19 @@ func netpollblock(pd *pollDesc, mode int32, waitio bool) bool { // set the gpp semaphore to pdWait for { - old := *gpp - if old == pdReady { - *gpp = 0 + // Consume notification if already ready. + if atomic.Casuintptr(gpp, pdReady, 0) { return true } - if old != 0 { - throw("runtime: double wait") - } if atomic.Casuintptr(gpp, 0, pdWait) { break } + + // Double check that this isn't corrupt; otherwise we'd loop + // forever. + if v := atomic.Loaduintptr(gpp); v != pdReady && v != 0 { + throw("runtime: double wait") + } } // need to recheck error states after setting gpp to pdWait @@ -452,7 +461,7 @@ func netpollunblock(pd *pollDesc, mode int32, ioready bool) *g { } for { - old := *gpp + old := atomic.Loaduintptr(gpp) if old == pdReady { return nil } -- cgit v1.2.3-54-g00ecf