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Thu, 21 Nov 2024 07:59:39 -0800 (PST) Message-ID: <8a4d1e32-ea48-491e-8ea2-c8cf4c1f934c@citrix.com> Date: Thu, 21 Nov 2024 15:59:37 +0000 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] xen/bitops: Fix break with in a for_each_set_bit() loop To: Frediano Ziglio Cc: Xen-devel , Jan Beulich , =?UTF-8?Q?Roger_Pau_Monn=C3=A9?= , Stefano Stabellini , Julien Grall References: <20241121145000.3107723-1-andrew.cooper3@citrix.com> Content-Language: en-GB From: Andrew Cooper Autocrypt: addr=andrew.cooper3@citrix.com; keydata= xsFNBFLhNn8BEADVhE+Hb8i0GV6mihnnr/uiQQdPF8kUoFzCOPXkf7jQ5sLYeJa0cQi6Penp VtiFYznTairnVsN5J+ujSTIb+OlMSJUWV4opS7WVNnxHbFTPYZVQ3erv7NKc2iVizCRZ2Kxn srM1oPXWRic8BIAdYOKOloF2300SL/bIpeD+x7h3w9B/qez7nOin5NzkxgFoaUeIal12pXSR Q354FKFoy6Vh96gc4VRqte3jw8mPuJQpfws+Pb+swvSf/i1q1+1I4jsRQQh2m6OTADHIqg2E ofTYAEh7R5HfPx0EXoEDMdRjOeKn8+vvkAwhviWXTHlG3R1QkbE5M/oywnZ83udJmi+lxjJ5 YhQ5IzomvJ16H0Bq+TLyVLO/VRksp1VR9HxCzItLNCS8PdpYYz5TC204ViycobYU65WMpzWe LFAGn8jSS25XIpqv0Y9k87dLbctKKA14Ifw2kq5OIVu2FuX+3i446JOa2vpCI9GcjCzi3oHV e00bzYiHMIl0FICrNJU0Kjho8pdo0m2uxkn6SYEpogAy9pnatUlO+erL4LqFUO7GXSdBRbw5 gNt25XTLdSFuZtMxkY3tq8MFss5QnjhehCVPEpE6y9ZjI4XB8ad1G4oBHVGK5LMsvg22PfMJ ISWFSHoF/B5+lHkCKWkFxZ0gZn33ju5n6/FOdEx4B8cMJt+cWwARAQABzSlBbmRyZXcgQ29v cGVyIDxhbmRyZXcuY29vcGVyM0BjaXRyaXguY29tPsLBegQTAQgAJAIbAwULCQgHAwUVCgkI CwUWAgMBAAIeAQIXgAUCWKD95wIZAQAKCRBlw/kGpdefoHbdD/9AIoR3k6fKl+RFiFpyAhvO 59ttDFI7nIAnlYngev2XUR3acFElJATHSDO0ju+hqWqAb8kVijXLops0gOfqt3VPZq9cuHlh IMDquatGLzAadfFx2eQYIYT+FYuMoPZy/aTUazmJIDVxP7L383grjIkn+7tAv+qeDfE+txL4 SAm1UHNvmdfgL2/lcmL3xRh7sub3nJilM93RWX1Pe5LBSDXO45uzCGEdst6uSlzYR/MEr+5Z JQQ32JV64zwvf/aKaagSQSQMYNX9JFgfZ3TKWC1KJQbX5ssoX/5hNLqxMcZV3TN7kU8I3kjK mPec9+1nECOjjJSO/h4P0sBZyIUGfguwzhEeGf4sMCuSEM4xjCnwiBwftR17sr0spYcOpqET ZGcAmyYcNjy6CYadNCnfR40vhhWuCfNCBzWnUW0lFoo12wb0YnzoOLjvfD6OL3JjIUJNOmJy RCsJ5IA/Iz33RhSVRmROu+TztwuThClw63g7+hoyewv7BemKyuU6FTVhjjW+XUWmS/FzknSi dAG+insr0746cTPpSkGl3KAXeWDGJzve7/SBBfyznWCMGaf8E2P1oOdIZRxHgWj0zNr1+ooF /PzgLPiCI4OMUttTlEKChgbUTQ+5o0P080JojqfXwbPAyumbaYcQNiH1/xYbJdOFSiBv9rpt TQTBLzDKXok86M7BTQRS4TZ/ARAAkgqudHsp+hd82UVkvgnlqZjzz2vyrYfz7bkPtXaGb9H4 Rfo7mQsEQavEBdWWjbga6eMnDqtu+FC+qeTGYebToxEyp2lKDSoAsvt8w82tIlP/EbmRbDVn 7bhjBlfRcFjVYw8uVDPptT0TV47vpoCVkTwcyb6OltJrvg/QzV9f07DJswuda1JH3/qvYu0p vjPnYvCq4NsqY2XSdAJ02HrdYPFtNyPEntu1n1KK+gJrstjtw7KsZ4ygXYrsm/oCBiVW/OgU g/XIlGErkrxe4vQvJyVwg6YH653YTX5hLLUEL1NS4TCo47RP+wi6y+TnuAL36UtK/uFyEuPy wwrDVcC4cIFhYSfsO0BumEI65yu7a8aHbGfq2lW251UcoU48Z27ZUUZd2Dr6O/n8poQHbaTd 6bJJSjzGGHZVbRP9UQ3lkmkmc0+XCHmj5WhwNNYjgbbmML7y0fsJT5RgvefAIFfHBg7fTY/i kBEimoUsTEQz+N4hbKwo1hULfVxDJStE4sbPhjbsPCrlXf6W9CxSyQ0qmZ2bXsLQYRj2xqd1 bpA+1o1j2N4/au1R/uSiUFjewJdT/LX1EklKDcQwpk06Af/N7VZtSfEJeRV04unbsKVXWZAk uAJyDDKN99ziC0Wz5kcPyVD1HNf8bgaqGDzrv3TfYjwqayRFcMf7xJaL9xXedMcAEQEAAcLB XwQYAQgACQUCUuE2fwIbDAAKCRBlw/kGpdefoG4XEACD1Qf/er8EA7g23HMxYWd3FXHThrVQ HgiGdk5Yh632vjOm9L4sd/GCEACVQKjsu98e8o3ysitFlznEns5EAAXEbITrgKWXDDUWGYxd pnjj2u+GkVdsOAGk0kxczX6s+VRBhpbBI2PWnOsRJgU2n10PZ3mZD4Xu9kU2IXYmuW+e5KCA vTArRUdCrAtIa1k01sPipPPw6dfxx2e5asy21YOytzxuWFfJTGnVxZZSCyLUO83sh6OZhJkk b9rxL9wPmpN/t2IPaEKoAc0FTQZS36wAMOXkBh24PQ9gaLJvfPKpNzGD8XWR5HHF0NLIJhgg 4ZlEXQ2fVp3XrtocHqhu4UZR4koCijgB8sB7Tb0GCpwK+C4UePdFLfhKyRdSXuvY3AHJd4CP 4JzW0Bzq/WXY3XMOzUTYApGQpnUpdOmuQSfpV9MQO+/jo7r6yPbxT7CwRS5dcQPzUiuHLK9i nvjREdh84qycnx0/6dDroYhp0DFv4udxuAvt1h4wGwTPRQZerSm4xaYegEFusyhbZrI0U9tJ B8WrhBLXDiYlyJT6zOV2yZFuW47VrLsjYnHwn27hmxTC/7tvG3euCklmkn9Sl9IAKFu29RSo d5bD8kMSCYsTqtTfT6W4A3qHGvIDta3ptLYpIAOD2sY3GYq2nf3Bbzx81wZK14JdDDHUX2Rs 6+ahAA== In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 21/11/2024 3:19 pm, Frediano Ziglio wrote: > On Thu, Nov 21, 2024 at 2:50 PM Andrew Cooper wrote: >> for_each_set_bit()'s use of a double for loop had an accidental bug with a >> break in the inner loop leading to an infinite outer loop. >> >> Adjust for_each_set_bit() to avoid this behaviour, and add extend >> test_for_each_set_bit() with a test case for this. >> >> Fixes: ed26376f20bf ("xen/bitops: Introduce for_each_set_bit()") >> Signed-off-by: Andrew Cooper >> --- >> CC: Jan Beulich >> CC: Roger Pau Monné >> CC: Frediano Ziglio >> CC: Stefano Stabellini >> CC: Julien Grall >> >> Both GCC and Clang seem happy with this, even at -O1: >> >> https://godbolt.org/z/o6ohjrzsY >> --- >> xen/common/bitops.c | 16 ++++++++++++++++ >> xen/include/xen/bitops.h | 2 +- >> 2 files changed, 17 insertions(+), 1 deletion(-) >> >> diff --git a/xen/common/bitops.c b/xen/common/bitops.c >> index 91ae961440af..0edd62d25c28 100644 >> --- a/xen/common/bitops.c >> +++ b/xen/common/bitops.c >> @@ -110,6 +110,22 @@ static void __init test_for_each_set_bit(void) >> >> if ( ull != ull_res ) >> panic("for_each_set_bit(uint64) expected %#"PRIx64", got %#"PRIx64"\n", ull, ull_res); >> + >> + /* Check that we break from the middle of the loop */ >> + ui = HIDE(0x80001008U); >> + ui_res = 0; >> + for_each_set_bit ( i, ui ) >> + { >> + static __initdata unsigned int count; >> + >> + if ( count++ > 1 ) >> + break; >> + >> + ui_res |= 1U << i; >> + } >> + >> + if ( ui_res != 0x1008 ) >> + panic("for_each_set_bit(break) expected 0x1008, got %#x\n", ui_res); >> } >> >> static void __init test_multiple_bits_set(void) >> diff --git a/xen/include/xen/bitops.h b/xen/include/xen/bitops.h >> index 79615fb89d04..448b2d3e0937 100644 >> --- a/xen/include/xen/bitops.h >> +++ b/xen/include/xen/bitops.h >> @@ -299,7 +299,7 @@ static always_inline attr_const unsigned int fls64(uint64_t x) >> * A copy of @val is taken internally. >> */ >> #define for_each_set_bit(iter, val) \ >> - for ( typeof(val) __v = (val); __v; ) \ >> + for ( typeof(val) __v = (val); __v; __v = 0 ) \ >> for ( unsigned int (iter); \ >> __v && ((iter) = ffs_g(__v) - 1, true); \ >> __v &= __v - 1 ) >> >> base-commit: e0058760a0c7935ad0690d8b9babb9050eceedf0 > Not a fun of static variables but it's just in the test, Oh, I guess I can do it with just a local variable.  Incremental diff is: @@ -86,7 +86,7 @@ static void __init test_fls(void)    static void __init test_for_each_set_bit(void)  { -    unsigned int  ui,  ui_res = 0; +    unsigned int  ui,  ui_res = 0, tmp;      unsigned long ul,  ul_res = 0;      uint64_t      ull, ull_res = 0;   @@ -113,12 +113,11 @@ static void __init test_for_each_set_bit(void)        /* Check that we break from the middle of the loop */      ui = HIDE(0x80001008U); +    tmp = 0;      ui_res = 0;      for_each_set_bit ( i, ui )      { -        static __initdata unsigned int count; - -        if ( count++ > 1 ) +        if ( tmp++ > 1 )              break;            ui_res |= 1U << i; > Reviewed-by: Frediano Ziglio Thanks.  This turns out to be a whole lot easier than I was fearing. ~Andrew