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x86/membarrier: Get rid of a dubious optimization 

sync_core_before_usermode() had an incorrect optimization.  If the kernel
returns from an interrupt, it can get to usermode without IRET. It just has
to schedule to a different task in the same mm and do SYSRET.  Fortunately,
there were no callers of sync_core_before_usermode() that could have had
in_irq() or in_nmi() equal to true, because it's only ever called from the
scheduler.

While at it, clarify a related comment.

Fixes: 70216e18e5 ("membarrier: Provide core serializing command, *_SYNC_CORE")
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/5afc7632be1422f91eaf7611aaaa1b5b8580a086.1607058304.git.luto@kernel.org
This commit is contained in:
Andy Lutomirski 2020-12-03 21:07:03 -08:00 committed by Thomas Gleixner
parent 0477e92881
commit a493d1ca1a
2 changed files with 13 additions and 6 deletions
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9
arch/x86/include/asm/sync_core.h
View File

@ -98,12 +98,13 @@ static inline void sync_core_before_usermode(void)
/* With PTI, we unconditionally serialize before running user code. */
if (static_cpu_has(X86_FEATURE_PTI))
return;
/*
* Return from interrupt and NMI is done through iret, which is core
* serializing.
* Even if we're in an interrupt, we might reschedule before returning,
* in which case we could switch to a different thread in the same mm
* and return using SYSRET or SYSEXIT. Instead of trying to keep
* track of our need to sync the core, just sync right away.
*/
if (in_irq() || in_nmi())
return;
sync_core();
}

10
arch/x86/mm/tlb.c
View File

@ -474,8 +474,14 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
/*
* The membarrier system call requires a full memory barrier and
* core serialization before returning to user-space, after
* storing to rq->curr. Writing to CR3 provides that full
* memory barrier and core serializing instruction.
* storing to rq->curr, when changing mm. This is because
* membarrier() sends IPIs to all CPUs that are in the target mm
* to make them issue memory barriers. However, if another CPU
* switches to/from the target mm concurrently with
* membarrier(), it can cause that CPU not to receive an IPI
* when it really should issue a memory barrier. Writing to CR3
* provides that full memory barrier and core serializing
* instruction.
*/
if (real_prev == next) {
VM_WARN_ON(this_cpu_read(cpu_tlbstate.ctxs[prev_asid].ctx_id) !=