From: Yury Norov <ynorov@xxxxxxxxxxxxxxxxxx> Subject: lib/bitmap.c: make bitmap_parselist() thread-safe and much faster Current implementation of bitmap_parselist() uses a static variable to save local state while setting bits in the bitmap. It is obviously wrong if we assume execution in multiprocessor environment. Fortunately, it's possible to rewrite this portion of code to avoid using the static variable. It is also possible to set bits in the mask per-range with bitmap_set(), not per-bit, as it is implemented now, with set_bit(); which is way faster. The important side effect of this change is that setting bits in this function from now is not per-bit atomic and less memory-ordered. This is because set_bit() guarantees the order of memory accesses, while bitmap_set() does not. I think that it is the advantage of the new approach, because the bitmap_parselist() is intended to initialise bit arrays, and user should protect the whole bitmap during initialisation if needed. So protecting individual bits looks expensive and useless. Also, other range-oriented functions in lib/bitmap.c don't worry much about atomicity. With all that, setting 2k bits in map with the pattern like 0-2047:128/256 becomes ~50 times faster after applying the patch in my testing environment (arm64 hosted on qemu). The second patch of the series adds the test for bitmap_parselist(). It's not intended to cover all tricky cases, just to make sure that I didn't screw up during rework. Link: http://lkml.kernel.org/r/20170807225438.16161-1-ynorov@xxxxxxxxxxxxxxxxxx Signed-off-by: Yury Norov <ynorov@xxxxxxxxxxxxxxxxxx> Cc: Noam Camus <noamca@xxxxxxxxxxxx> Cc: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx> Cc: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx> Cc: Mauro Carvalho Chehab <mchehab@xxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- lib/bitmap.c | 18 ++++++------------ 1 file changed, 6 insertions(+), 12 deletions(-) diff -puN lib/bitmap.c~lib-make-bitmap_parselist-thread-safe-and-much-faster lib/bitmap.c --- a/lib/bitmap.c~lib-make-bitmap_parselist-thread-safe-and-much-faster +++ a/lib/bitmap.c @@ -513,7 +513,7 @@ static int __bitmap_parselist(const char int nmaskbits) { unsigned int a, b, old_a, old_b; - unsigned int group_size, used_size; + unsigned int group_size, used_size, off; int c, old_c, totaldigits, ndigits; const char __user __force *ubuf = (const char __user __force *)buf; int at_start, in_range, in_partial_range; @@ -599,6 +599,8 @@ static int __bitmap_parselist(const char a = old_a; b = old_b; old_a = old_b = 0; + } else { + used_size = group_size = b - a + 1; } /* if no digit is after '-', it's wrong*/ if (at_start && in_range) @@ -608,17 +610,9 @@ static int __bitmap_parselist(const char if (b >= nmaskbits) return -ERANGE; while (a <= b) { - if (in_partial_range) { - static int pos_in_group = 1; - - if (pos_in_group <= used_size) - set_bit(a, maskp); - - if (a == b || ++pos_in_group > group_size) - pos_in_group = 1; - } else - set_bit(a, maskp); - a++; + off = min(b - a + 1, used_size); + bitmap_set(maskp, a, off); + a += group_size; } } while (buflen && c == ','); return 0; _ -- To unsubscribe from this list: send the line "unsubscribe mm-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
