219240 2020-11-21 16:47:44 -0800 Calling waitForAndDispatchImmediately<M> on a loop fails when multiple M messages arrive simultaneously 2020-11-30 10:44:11 -0800 1 1 1 Unclassified WebKit WebKit2 WebKit Nightly Build Unspecified Unspecified RESOLVED FIXED InRadar P2 Normal --- 219091 1 wenson_hsieh wenson_hsieh cdumez ggaren sabouhallawa thorton webkit-bug-importer oldest_to_newest 1709797 0 wenson_hsieh 2020-11-21 16:47:44 -0800 Fix a race condition in IPC logic, wherein a client that calls waitForAndDispatchImmediately<M>() n times may time out when receiving n messages of type M. ...more concretely: RemoteImageBufferProxy sync-waits for consecutive `RemoteRenderingBackendProxy::DidFlush` messages, and finishes sync-waiting when the DidFlush message containing the expected flush identifier arrives. The following may occur: (Main thread): waitForAndDispatchImmediately<RemoteRenderingBackendProxy::DidFlush>() (IPC thread): receives the first DidFlush message (IPC thread): sees that the message name matches that of m_waitingForMessage, and signals the main thread (Main thread): sees that m_waitingForMessage now has a decoder, and nulls out m_waitingForMessage (Main thread): calls into RemoteRenderingBackendProxy::didFlush() and returns from waitForAndDispatchImmediately (IPC thread): receives the second DidFlush message (IPC thread): sees that m_waitingForMessage is null (Main thread): calls into RemoteRenderingBackendProxy::didFlush() and returns from waitForAndDispatchImmediately (Main thread): sees that the flush is still pending, so we call waitForAndDispatchImmediately<RemoteRenderingBackendProxy::DidFlush>() again (Main thread): sees that the incoming messages queue doesn't contain DidFlush, so we wait (IPC thread): enqueues the incoming DidFlush message onto the main thread (via enqueueIncomingMessage) ...and now the main thread will time out waiting for the RemoteRenderingBackendProxy::DidFlush which is currently sitting in the incoming messages queue :| 1709936 1 414812 wenson_hsieh 2020-11-23 12:54:16 -0800 Created attachment 414812 Patch 1709960 2 414812 cdumez 2020-11-23 19:18:50 -0800 Comment on attachment 414812 Patch r=me assuming the bots are happy. 1710210 3 414812 wenson_hsieh 2020-11-26 20:40:34 -0800 Comment on attachment 414812 Patch Thanks for the review! 1710211 4 ews-feeder 2020-11-26 20:42:56 -0800 Committed r270180: <https://trac.webkit.org/changeset/270180> All reviewed patches have been landed. Closing bug and clearing flags on attachment 414812. 1710212 5 webkit-bug-importer 2020-11-26 20:43:24 -0800 <rdar://problem/71757243> 1710663 6 414812 ggaren 2020-11-30 10:23:53 -0800 Comment on attachment 414812 Patch View in context: https://bugs.webkit.org/attachment.cgi?id=414812&action=review > Source/WebKit/Platform/IPC/Connection.cpp:553 > + if (hasIncomingSynchronousMessage && waitForOptions.contains(WaitForOption::InterruptWaitingIfSyncMessageArrives)) { Since we needed to hold a lock while computing hasIncomingSynchronousMessage, it seems like it must be a bug to check hasIncomingSynchronousMessage after dropping the lock. Between dropping the lock and checking hasIncomingSynchronousMessage, the true value of hasIncomingSynchronousMessage may have changed (because a new incoming message may have arrived). 1710666 7 414812 ggaren 2020-11-30 10:26:45 -0800 Comment on attachment 414812 Patch View in context: https://bugs.webkit.org/attachment.cgi?id=414812&action=review >> Source/WebKit/Platform/IPC/Connection.cpp:553 >> + if (hasIncomingSynchronousMessage && waitForOptions.contains(WaitForOption::InterruptWaitingIfSyncMessageArrives)) { > > Since we needed to hold a lock while computing hasIncomingSynchronousMessage, it seems like it must be a bug to check hasIncomingSynchronousMessage after dropping the lock. Between dropping the lock and checking hasIncomingSynchronousMessage, the true value of hasIncomingSynchronousMessage may have changed (because a new incoming message may have arrived). But I guess this is OK because there is only one IPC thread, and we are running on it, and we know that only the IPC thread adds to m_incomingMessages? 1710669 8 cdumez 2020-11-30 10:44:11 -0800 (In reply to Geoffrey Garen from comment #7) > Comment on attachment 414812 [details] > Patch > > View in context: > https://bugs.webkit.org/attachment.cgi?id=414812&action=review > > >> Source/WebKit/Platform/IPC/Connection.cpp:553 > >> + if (hasIncomingSynchronousMessage && waitForOptions.contains(WaitForOption::InterruptWaitingIfSyncMessageArrives)) { > > > > Since we needed to hold a lock while computing hasIncomingSynchronousMessage, it seems like it must be a bug to check hasIncomingSynchronousMessage after dropping the lock. Between dropping the lock and checking hasIncomingSynchronousMessage, the true value of hasIncomingSynchronousMessage may have changed (because a new incoming message may have arrived). > > But I guess this is OK because there is only one IPC thread, and we are > running on it, and we know that only the IPC thread adds to > m_incomingMessages? 2 things: a. This is not new behavior in this patch b. Yes, it is my understanding that incoming messages only get enqueued by the IPC thread and we are on the IPC thread here. The reason we have a lock is because the main thread dequeues those messages. 414812 2020-11-23 12:54:16 -0800 2020-11-26 20:42:57 -0800 Patch bug-219240-20201123125415.patch text/plain 7870 wenson_hsieh U3VidmVyc2lvbiBSZXZpc2lvbjogMjcwMTY1CmRpZmYgLS1naXQgYS9Tb3VyY2UvV2ViS2l0L0No YW5nZUxvZyBiL1NvdXJjZS9XZWJLaXQvQ2hhbmdlTG9nCmluZGV4IGM1M2QyNGIyY2NlZWQ5MDIx YzdhNTRjNTI0YzM4ODhiMTBjZDNiMzAuLmJiMTk0MDA5YzAxMDNjYjRlMTViMzQ1NTJiOWVjMGY0 NGIyMDQ1ODkgMTAwNjQ0Ci0tLSBhL1NvdXJjZS9XZWJLaXQvQ2hhbmdlTG9nCisrKyBiL1NvdXJj ZS9XZWJLaXQvQ2hhbmdlTG9nCkBAIC0xLDMgKzEsNTAgQEAKKzIwMjAtMTEtMjMgIFdlbnNvbiBI c2llaCAgPHdlbnNvbl9oc2llaEBhcHBsZS5jb20+CisKKyAgICAgICAgQ2FsbGluZyB3YWl0Rm9y QW5kRGlzcGF0Y2hJbW1lZGlhdGVseTxNPiBvbiBhIGxvb3AgZmFpbHMgd2hlbiBtdWx0aXBsZSBN IG1lc3NhZ2VzIGFycml2ZSBzaW11bHRhbmVvdXNseQorICAgICAgICBodHRwczovL2J1Z3Mud2Vi a2l0Lm9yZy9zaG93X2J1Zy5jZ2k/aWQ9MjE5MjQwCisKKyAgICAgICAgUmV2aWV3ZWQgYnkgTk9C T0RZIChPT1BTISkuCisKKyAgICAgICAgRml4ZXMgYSByYWNlIHRoYXQgbWF5IG9jY3VyIHdoZW4g 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