https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109933
Bug ID: 109933
Summary: __atomic_test_and_set is broken for BIG ENDIAN riscv
targets
Product: gcc
Version: 12.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: rory.bolt at gmail dot com
Target Milestone: ---
Target: riscv64be-unknown-linux-gnu
Created attachment 55136
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=55136&action=edit
Simple test program illustrating the bug
PLEASE NOTE: this bug is in reference to BIG ENDIAN riscv targets...
Although reported on riscv64be, it most likely exists for 32 bit targets too.
Please see the attached test program.
The issue appears to be that the value atomically or'ed should be 0x01000000
not 1 for big endian systems, since we want the non-zero value in the lowest
memory address and the amoor.w command is writing a 32 bit value to memory.
The current (broken) implementation will pass a simple unit test, the problem
only manifests itself on a call to __atomic_test_and_set AFTER a previous call
to __atomic_test_and_set to the same address and a __atomic_clear to the same
address.
This problem originally manifested itself in the linux libcap library, which
uses __atomic_test_and_set in conjunction with __atomic_clear to implement a
mutex to guard the manipulation of capabilities.
