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Commit 68fc378c authored by Michael Ellerman's avatar Michael Ellerman
Browse files

Revert "powerpc/tm: Abort syscalls in active transactions" 



This reverts commit feba4036.

Although the principle of this change is good, the implementation has a
few issues.

Firstly we can sometimes fail to abort a syscall because r12 may have
been clobbered by C code if we went down the virtual CPU accounting
path, or if syscall tracing was enabled.

Secondly we have decided that it is safer to abort the syscall even
earlier in the syscall entry path, so that we avoid the syscall tracing
path when we are transactional.

So that we have time to thoroughly test those changes we have decided to
revert this for this merge window and will merge the fixed version in
the next window.

NB. Rather than reverting the selftest we just drop tm-syscall from
TEST_PROGS so that it's not run by default.

Fixes: feba4036 ("powerpc/tm: Abort syscalls in active transactions")
Signed-off-by: default avatarMichael Ellerman <mpe@ellerman.id.au>
parent d33047fd

Documentation/powerpc/transactional_memory.txt

+16 −16
Original line number Diff line number Diff line
@@ -74,23 +74,22 @@ Causes of transaction aborts 
Syscalls

========



Syscalls made from within an active transaction will not be performed and the

transaction will be doomed by the kernel with the failure code TM_CAUSE_SYSCALL

| TM_CAUSE_PERSISTENT.

Performing syscalls from within transaction is not recommended, and can lead

to unpredictable results.



Syscalls made from within a suspended transaction are performed as normal and

the transaction is not explicitly doomed by the kernel.  However, what the

kernel does to perform the syscall may result in the transaction being doomed

by the hardware.  The syscall is performed in suspended mode so any side

effects will be persistent, independent of transaction success or failure.  No

guarantees are provided by the kernel about which syscalls will affect

transaction success.

Syscalls do not by design abort transactions, but beware: The kernel code will

not be running in transactional state.  The effect of syscalls will always

remain visible, but depending on the call they may abort your transaction as a

side-effect, read soon-to-be-aborted transactional data that should not remain

invisible, etc.  If you constantly retry a transaction that constantly aborts

itself by calling a syscall, you'll have a livelock & make no progress.



Care must be taken when relying on syscalls to abort during active transactions

if the calls are made via a library.  Libraries may cache values (which may

give the appearance of success) or perform operations that cause transaction

failure before entering the kernel (which may produce different failure codes).

Examples are glibc's getpid() and lazy symbol resolution.

Simple syscalls (e.g. sigprocmask()) "could" be OK.  Even things like write()

from, say, printf() should be OK as long as the kernel does not access any

memory that was accessed transactionally.



Consider any syscalls that happen to work as debug-only -- not recommended for

production use.  Best to queue them up till after the transaction is over.





Signals
@@ -177,7 +176,8 @@ kernel aborted a transaction: 
 TM_CAUSE_RESCHED       Thread was rescheduled.

 TM_CAUSE_TLBI          Software TLB invalid.

 TM_CAUSE_FAC_UNAV      FP/VEC/VSX unavailable trap.

 TM_CAUSE_SYSCALL       Syscall from active transaction.

 TM_CAUSE_SYSCALL       Currently unused; future syscalls that must abort

                        transactions for consistency will use this.

 TM_CAUSE_SIGNAL        Signal delivered.

 TM_CAUSE_MISC          Currently unused.

 TM_CAUSE_ALIGNMENT     Alignment fault.

arch/powerpc/include/uapi/asm/tm.h

+1 −1
Original line number Diff line number Diff line
@@ -11,7 +11,7 @@ 
#define TM_CAUSE_RESCHED	0xde

#define TM_CAUSE_TLBI		0xdc

#define TM_CAUSE_FAC_UNAV	0xda

#define TM_CAUSE_SYSCALL	0xd8

#define TM_CAUSE_SYSCALL	0xd8  /* future use */

#define TM_CAUSE_MISC		0xd6  /* future use */

#define TM_CAUSE_SIGNAL		0xd4

#define TM_CAUSE_ALIGNMENT	0xd2

arch/powerpc/kernel/entry_64.S

+0 −19
Original line number Diff line number Diff line
@@ -34,7 +34,6 @@ 
#include <asm/ftrace.h>

#include <asm/hw_irq.h>

#include <asm/context_tracking.h>

#include <asm/tm.h>



/*

 * System calls.
@@ -146,24 +145,6 @@ END_FW_FTR_SECTION_IFSET(FW_FEATURE_SPLPAR) 
	andi.	r11,r10,_TIF_SYSCALL_DOTRACE

	bne	syscall_dotrace

.Lsyscall_dotrace_cont:

#ifdef CONFIG_PPC_TRANSACTIONAL_MEM

BEGIN_FTR_SECTION

	b	1f

END_FTR_SECTION_IFCLR(CPU_FTR_TM)

	extrdi.	r11, r12, 1, (63-MSR_TS_T_LG) /* transaction active? */

	beq+	1f



	/* Doom the transaction and don't perform the syscall: */

	mfmsr	r11

	li	r12, 1

	rldimi	r11, r12, MSR_TM_LG, 63-MSR_TM_LG

	mtmsrd	r11, 0

	li	r11, (TM_CAUSE_SYSCALL|TM_CAUSE_PERSISTENT)

	TABORT(R11)



	b	.Lsyscall_exit

1:

#endif

	cmpldi	0,r0,NR_syscalls

	bge-	syscall_enosys

tools/testing/selftests/powerpc/tm/Makefile

+1 −1
Original line number Diff line number Diff line 
TEST_PROGS := tm-resched-dscr tm-syscall

TEST_PROGS := tm-resched-dscr



all: $(TEST_PROGS)