public inbox for gcc-cvs@sourceware.org
help / color / mirror / Atom feed
* [gcc r14-2844] ada: Fix race condition in protected entry call
@ 2023-07-28 7:31 Marc Poulhi?s
0 siblings, 0 replies; only message in thread
From: Marc Poulhi?s @ 2023-07-28 7:31 UTC (permalink / raw)
To: gcc-cvs
https://gcc.gnu.org/g:5bd09a25b342f980f0596c9e354c54a4c635ec65
commit r14-2844-g5bd09a25b342f980f0596c9e354c54a4c635ec65
Author: Ronan Desplanques <desplanques@adacore.com>
Date: Tue Jul 11 17:57:14 2023 +0200
ada: Fix race condition in protected entry call
This patch only affects the single-entry implementation of protected
objects.
Before this patch, there was a race condition where a task that
called an entry could put itself to sleep right after another task
had executed the entry as a proxy and signalled the not-yet-waiting
first task, which caused the first task to enter a deadlock.
Note that this race condition has been identified and fixed before
for the implementations of the run-time that live under hie/.
This patch reworks the locking sequence so that it is closer to the
one that's used in the multiple-entry implementation of protected
objects. The code for the multiple-entry implementation is spread
across multiple subprograms. To draw a parallel with the section
this patch modifies, one can read the following subprograms:
- System.Tasking.Protected_Objects.Operations.Protected_Entry_Call
- System.Tasking.Entry_Calls.Wait_For_Completion
- System.Tasking.Entry_Calls.Check_Pending_Actions_For_Entry_Call
This patch also adds a comment that explicitly states the locking
constraint that must hold in the affected section.
gcc/ada/
* libgnarl/s-tposen.adb: Fix race condition. Add comment to justify
the locking timing.
Diff:
---
gcc/ada/libgnarl/s-tposen.adb | 10 ++++++++--
1 file changed, 8 insertions(+), 2 deletions(-)
diff --git a/gcc/ada/libgnarl/s-tposen.adb b/gcc/ada/libgnarl/s-tposen.adb
index 9dff6619295..a7447b9e2af 100644
--- a/gcc/ada/libgnarl/s-tposen.adb
+++ b/gcc/ada/libgnarl/s-tposen.adb
@@ -345,11 +345,17 @@ package body System.Tasking.Protected_Objects.Single_Entry is
pragma Assert (Entry_Call.State /= Cancelled);
+ -- Note that we need to acquire Self_Id's lock before checking the value
+ -- of Entry_Call.State, even though the latter is specified as atomic
+ -- with a pragma. If we didn't, another task could execute the entry on
+ -- our behalf right between the check of Entry_Call.State and the call
+ -- to Wait_For_Completion, and that would cause a deadlock.
+
+ STPO.Write_Lock (Self_Id);
if Entry_Call.State /= Done then
- STPO.Write_Lock (Self_Id);
Wait_For_Completion (Entry_Call'Access);
- STPO.Unlock (Self_Id);
end if;
+ STPO.Unlock (Self_Id);
Check_Exception (Self_Id, Entry_Call'Access);
end Protected_Single_Entry_Call;
^ permalink raw reply [flat|nested] only message in thread
only message in thread, other threads:[~2023-07-28 7:31 UTC | newest]
Thread overview: (only message) (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2023-07-28 7:31 [gcc r14-2844] ada: Fix race condition in protected entry call Marc Poulhi?s
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).