[tip: core/entry] powerpc: Avoid discarding flags in system_call_exception()

[tip-bot2 for Mark Rutland]

The following commit has been merged into the core/entry branch of tip:

Commit-ID:     51ed65dcfd9c61a15920a40178d471d236a7514c
Gitweb:        https://git.kernel.org/tip/51ed65dcfd9c61a15920a40178d471d236a7514c
Author:        Mark Rutland <mark.rutland@xxxxxxx>
AuthorDate:    Wed, 17 Nov 2021 16:30:47 
Committer:     Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitterDate: Fri, 26 Nov 2021 21:20:13 +01:00

powerpc: Avoid discarding flags in system_call_exception()

Some thread flags can be set remotely, and so even when IRQs are
disabled, the flags can change under our feet. Thus, when setting flags
we must use an atomic operation rather than a plain read-modify-write
sequence, as a plain read-modify-write may discard flags which are
concurrently set by a remote thread, e.g.

	// task A			// task B
	tmp = A->thread_info.flags;
					set_tsk_thread_flag(A, NEWFLAG_B);
	tmp |= NEWFLAG_A;
	A->thread_info.flags = tmp;

In arch/powerpc/kernel/interrupt.c's system_call_exception(), we set
_TIF_RESTOREALL in the thread info flags with a read-modify-write, which
may result in other flags being discarded.

Elsewhere in the file we use clear_bits() to atomically remove flag
bits, so let's use set_bits() here for consistency with those.

I presume there may be reasons (e.g. instrumentation) that prevent the
use of set_thread_flag() and clear_thread_flag() here, which would
otherwise be preferable.

Fixes: ae7aaecc3f2f78b7 ("powerpc/64s: system call rfscv workaround for TM bugs")
Signed-off-by: Mark Rutland <mark.rutland@xxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Eirik Fuller <efuller@xxxxxxxxxx>
Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Cc: Nicholas Piggin <npiggin@xxxxxxxxx>
Link: https://lore.kernel.org/r/20211117163050.53986-10-mark.rutland@xxxxxxx
---
 arch/powerpc/kernel/interrupt.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/powerpc/kernel/interrupt.c b/arch/powerpc/kernel/interrupt.c
index 835b626..1c821b7 100644
--- a/arch/powerpc/kernel/interrupt.c
+++ b/arch/powerpc/kernel/interrupt.c
@@ -148,7 +148,7 @@ notrace long system_call_exception(long r3, long r4, long r5,
 	 */
 	if (IS_ENABLED(CONFIG_PPC_TRANSACTIONAL_MEM) &&
 			unlikely(MSR_TM_TRANSACTIONAL(regs->msr)))
-		current_thread_info()->flags |= _TIF_RESTOREALL;
+		set_bits(_TIF_RESTOREALL, current_thread_info()->flags);
 
 	/*
 	 * If the system call was made with a transaction active, doom it and