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[2020-03-25 05:51 UTC] bugs at rudeirer dot wtf
Description:
------------
I observed several non-responding FPM worker processes using 100% CPU. Attaching gdb and printing a stack trace indicated that the issue might be with opcache. In /proc/PID/fd, it could be seen that all affected processes had opened a specific file. Disabling opcache or blacklisting the file worked around the problem.
By trial and error, I removed lines from the problematic file in order to produce a short test script. The code is not runnable any more, but it's small and still causes the problem. (In the original code, all referenced variables were properly defined.)
I originally encountered this problem while using PHP 7.3.14 as supplied by Debian, so I believe 7.3 is also affected. For testing, I downloaded and compiled PHP 7.4.4 in a clean Debian 10 chroot. The configure line was "./configure --enable-debug --prefix=/opt/php/7.4.4". No changes were made to php.ini or any other file. Saving the test script as "bug.php" and running it with "php -d zend_extension=opcache.so -d opcache.enable_cli=1 bug.php" exhibits the problematic behavior.
This is a stack trace obtained after letting php run the test script for a few seconds:
#0 0x00007f3c0717639b in zend_infer_ranges (op_array=0x7f3c07277008, ssa=0x7f3c07286028) at /home/build/php-7.4.4/ext/opcache/Optimizer/zend_inference.c:1815
#1 0x00007f3c0719268d in zend_ssa_inference (arena=0x7ffc6c37df10, op_array=0x7f3c07277008, script=0x7f3c07277000, ssa=0x7f3c07286028, optimization_level=2147401727) at /home/build/php-7.4.4/ext/opcache/Optimizer/zend_inference.c:4330
#2 0x00007f3c0715ee7a in zend_dfa_analyze_op_array (op_array=0x7f3c07277008, ctx=0x7ffc6c37df10, ssa=0x7f3c07286028) at /home/build/php-7.4.4/ext/opcache/Optimizer/dfa_pass.c:113
#3 0x00007f3c07142fdb in zend_optimize_script (script=0x7f3c07277000, optimization_level=2147401727, debug_level=0) at /home/build/php-7.4.4/ext/opcache/Optimizer/zend_optimizer.c:1382
#4 0x00007f3c071113f8 in cache_script_in_shared_memory (new_persistent_script=0x7f3c07277000, key=0x7f3c071d6da4 <accel_globals+484> "bug.php:2274224:2220784", key_length=23, from_shared_memory=0x7ffc6c37e220) at /home/build/php-7.4.4/ext/opcache/ZendAccelerator.c:1451
#5 0x00007f3c071132da in persistent_compile_file (file_handle=0x7ffc6c381810, type=8) at /home/build/php-7.4.4/ext/opcache/ZendAccelerator.c:2136
#6 0x0000563a74674bf5 in zend_execute_scripts (type=8, retval=0x0, file_count=3) at /home/build/php-7.4.4/Zend/zend.c:1659
#7 0x0000563a745dd9dd in php_execute_script (primary_file=0x7ffc6c381810) at /home/build/php-7.4.4/main/main.c:2617
#8 0x0000563a7474a481 in do_cli (argc=6, argv=0x563a767e78c0) at /home/build/php-7.4.4/sapi/cli/php_cli.c:961
#9 0x0000563a7474b451 in main (argc=6, argv=0x563a767e78c0) at /home/build/php-7.4.4/sapi/cli/php_cli.c:1356
Test script:
---------------
<?php
$limitPerRun = 10;
foreach ($foo as $bar)
{
$count = 0;
foreach ($runs as $run)
{
++$count;
if ($count >= $limitPerRun)
{
break;
}
}
foo($limitPerRun);
}
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Last updated: Sat Oct 25 05:00:02 2025 UTC |
@dmitry: You might want to take a look at this, I don't understand the details of the range inference algorithm well. I think the basic issues is that #15.CV3($count) = Pi<BB4>(#21.CV3($count) & RANGE[-- .. #17.CV0($limitPerRun) - 1]) narrows the range when $limitPerRun is 10, but later gets expanded to the full range, at which point we're doing to start incrementing the range one by one (because we're using a narrowing meet at this point). But I can't really tell what the root problem here is.