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s390/uaccess: limit number of retries for cmpxchg_user_key()

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cmpxchg_user_key() for byte and short values is implemented via a one word
cmpxchg loop. Give up trying to perform the cmpxchg if it fails too often
because of contention on the cache line. This ensures that the thread
cannot become stuck in the kernel.

Signed-off-by: Janis Schoetterl-Glausch <scgl@linux.ibm.com>
Link: https://lore.kernel.org/r/20221117100745.3253896-1-scgl@linux.ibm.com
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
This commit is contained in:
Janis Schoetterl-Glausch 2022-11-17 11:07:45 +01:00 committed by Heiko Carstens
parent 51098f0eb2
commit 739ad2e4e1
1 changed files with 25 additions and 8 deletions
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33
arch/s390/include/asm/uaccess.h
View file

@ -392,6 +392,8 @@ do { \
void __cmpxchg_user_key_called_with_bad_pointer(void);
#define CMPXCHG_USER_KEY_MAX_LOOPS 128
static __always_inline int __cmpxchg_user_key(unsigned long address, void *uval,
__uint128_t old, __uint128_t new,
unsigned long key, int size)
@ -401,6 +403,7 @@ static __always_inline int __cmpxchg_user_key(unsigned long address, void *uval,
switch (size) {
case 1: {
unsigned int prev, shift, mask, _old, _new;
unsigned long count;
shift = (3 ^ (address & 3)) << 3;
address ^= address & 3;
@ -410,6 +413,7 @@ static __always_inline int __cmpxchg_user_key(unsigned long address, void *uval,
asm volatile(
" spka 0(%[key])\n"
" sacf 256\n"
" llill %[count],%[max_loops]\n"
"0: l %[prev],%[address]\n"
"1: nr %[prev],%[mask]\n"
" xilf %[mask],0xffffffff\n"
@ -421,7 +425,8 @@ static __always_inline int __cmpxchg_user_key(unsigned long address, void *uval,
" xr %[tmp],%[prev]\n"
" xr %[new],%[tmp]\n"
" nr %[tmp],%[mask]\n"
" jz 2b\n"
" jnz 5f\n"
" brct %[count],2b\n"
"5: sacf 768\n"
" spka %[default_key]\n"
EX_TABLE_UA_LOAD_REG(0b, 5b, %[rc], %[prev])
@ -433,15 +438,20 @@ static __always_inline int __cmpxchg_user_key(unsigned long address, void *uval,
[address] "+Q" (*(int *)address),
[tmp] "+&d" (_old),
[new] "+&d" (_new),
[mask] "+&d" (mask)
: [key] "a" (key << 4),
[default_key] "J" (PAGE_DEFAULT_KEY)
[mask] "+&d" (mask),
[count] "=a" (count)
: [key] "%[count]" (key << 4),
[default_key] "J" (PAGE_DEFAULT_KEY),
[max_loops] "J" (CMPXCHG_USER_KEY_MAX_LOOPS)
: "memory", "cc");
*(unsigned char *)uval = prev >> shift;
if (!count)
rc = -EAGAIN;
return rc;
}
case 2: {
unsigned int prev, shift, mask, _old, _new;
unsigned long count;
shift = (2 ^ (address & 2)) << 3;
address ^= address & 2;
@ -451,6 +461,7 @@ static __always_inline int __cmpxchg_user_key(unsigned long address, void *uval,
asm volatile(
" spka 0(%[key])\n"
" sacf 256\n"
" llill %[count],%[max_loops]\n"
"0: l %[prev],%[address]\n"
"1: nr %[prev],%[mask]\n"
" xilf %[mask],0xffffffff\n"
@ -462,7 +473,8 @@ static __always_inline int __cmpxchg_user_key(unsigned long address, void *uval,
" xr %[tmp],%[prev]\n"
" xr %[new],%[tmp]\n"
" nr %[tmp],%[mask]\n"
" jz 2b\n"
" jnz 5f\n"
" brct %[count],2b\n"
"5: sacf 768\n"
" spka %[default_key]\n"
EX_TABLE_UA_LOAD_REG(0b, 5b, %[rc], %[prev])
@ -474,11 +486,15 @@ static __always_inline int __cmpxchg_user_key(unsigned long address, void *uval,
[address] "+Q" (*(int *)address),
[tmp] "+&d" (_old),
[new] "+&d" (_new),
[mask] "+&d" (mask)
: [key] "a" (key << 4),
[default_key] "J" (PAGE_DEFAULT_KEY)
[mask] "+&d" (mask),
[count] "=a" (count)
: [key] "%[count]" (key << 4),
[default_key] "J" (PAGE_DEFAULT_KEY),
[max_loops] "J" (CMPXCHG_USER_KEY_MAX_LOOPS)
: "memory", "cc");
*(unsigned short *)uval = prev >> shift;
if (!count)
rc = -EAGAIN;
return rc;
}
case 4: {
@ -568,6 +584,7 @@ static __always_inline int __cmpxchg_user_key(unsigned long address, void *uval,
*
* Return: 0: cmpxchg executed
* -EFAULT: an exception happened when trying to access *@ptr
* -EAGAIN: maxed out number of retries (byte and short only)
*/
#define cmpxchg_user_key(ptr, uval, old, new, key) \
({ \