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Subject[PATCH] x86/apic: Fix atomic update of offset in reserve_eilvt_offset
Date
The detection of atomic update failure in reserve_eilvt_offset is not correct.
The value, returned by atomic_cmpxchg should be compared to the old value
from the location to be updated. If these two are the same, then atomic update
succeeded and "eilvt_offsets[offset]" location is updated to "new" in an
atomic way. Otherwise, the atomic update failed and it should be retried with
the value from "eilvt_offsets[offset]" - exactly what atomic_try_cmpxchg does
in a correct and more optimal way.

Fixes: a68c439b1966c ("apic, x86: Check if EILVT APIC registers are available (AMD only)")
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
---
arch/x86/kernel/apic/apic.c | 5 ++---
1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/apic/apic.c b/arch/x86/kernel/apic/apic.c
index 20d9a604da7c..770557110051 100644
--- a/arch/x86/kernel/apic/apic.c
+++ b/arch/x86/kernel/apic/apic.c
@@ -422,10 +422,9 @@ static unsigned int reserve_eilvt_offset(int offset, unsigned int new)
if (vector && !eilvt_entry_is_changeable(vector, new))
/* may not change if vectors are different */
return rsvd;
- rsvd = atomic_cmpxchg(&eilvt_offsets[offset], rsvd, new);
- } while (rsvd != new);
+ } while (!atomic_try_cmpxchg(&eilvt_offsets[offset], &rsvd, new));

- rsvd &= ~APIC_EILVT_MASKED;
+ rsvd = new & ~APIC_EILVT_MASKED;
if (rsvd && rsvd != vector)
pr_info("LVT offset %d assigned for vector 0x%02x\n",
offset, rsvd);
--
2.39.2
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