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x86: don't call '->send_IPI_mask()' with an empty mask

As noted in 83d349f35e1ae72268c5104dbf9ab2ae635425d4 ("x86: don't send
an IPI to the empty set of CPU's"), some APIC's will be very unhappy
with an empty destination mask. That commit added a WARN_ON() for that
case, and avoided the resulting problem, but didn't fix the underlying
reason for why those empty mask cases happened.

This fixes that, by checking the result of 'cpumask_andnot()' of the
current CPU actually has any other CPU's left in the set of CPU's to be
sent a TLB flush, and not calling down to the IPI code if the mask is
empty.

The reason this started happening at all is that we started passing just
the CPU mask pointers around in commit 4595f9620 ("x86: change
flush_tlb_others to take a const struct cpumask"), and when we did that,
the cpumask was no longer thread-local.

Before that commit, flush_tlb_mm() used to create it's own copy of
'mm->cpu_vm_mask' and pass that copy down to the low-level flush
routines after having tested that it was not empty. But after changing
it to just pass down the CPU mask pointer, the lower level TLB flush
routines would now get a pointer to that 'mm->cpu_vm_mask', and that
could still change - and become empty - after the test due to other
CPU's having flushed their own TLB's.

See

http://bugzilla.kernel.org/show_bug.cgi?id=13933

for details.

Tested-by: Thomas Björnell <thomas.bjornell@gmail.com>
Cc: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Linus Torvalds b04e6373 f4b0373b

+10 -11
+10 -11
arch/x86/mm/tlb.c
··· 183 183 184 184 f->flush_mm = mm; 185 185 f->flush_va = va; 186 - cpumask_andnot(to_cpumask(f->flush_cpumask), 187 - cpumask, cpumask_of(smp_processor_id())); 186 + if (cpumask_andnot(to_cpumask(f->flush_cpumask), cpumask, cpumask_of(smp_processor_id()))) { 187 + /* 188 + * We have to send the IPI only to 189 + * CPUs affected. 190 + */ 191 + apic->send_IPI_mask(to_cpumask(f->flush_cpumask), 192 + INVALIDATE_TLB_VECTOR_START + sender); 188 193 189 - /* 190 - * We have to send the IPI only to 191 - * CPUs affected. 192 - */ 193 - apic->send_IPI_mask(to_cpumask(f->flush_cpumask), 194 - INVALIDATE_TLB_VECTOR_START + sender); 195 - 196 - while (!cpumask_empty(to_cpumask(f->flush_cpumask))) 197 - cpu_relax(); 194 + while (!cpumask_empty(to_cpumask(f->flush_cpumask))) 195 + cpu_relax(); 196 + } 198 197 199 198 f->flush_mm = NULL; 200 199 f->flush_va = 0;