___test_and_clear_bit(9)

🌸 ___test_and_clear_bit — NAME 💅

okay so bestie, gather round: ___test_and_clear_bit is literally the function that clears a bit and returns its old value. that's the whole vibe. clear it, but tell me what it was first. iconic behaviour honestly 🎀

it lives in the Kernel API Manual, which is a whole mood. and yes — count the underscores, there are three of them up front. do not come for me. 😭

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✨ SYNOPSIS ✨

bool ___test_and_clear_bit (unsigned long nr , volatile unsigned long
*addr );

so it hands you back a bool, takes a unsigned long nr, and a volatile unsigned long *addr. exactly like that. the spaces are part of the assignment, bestie, we don't touch them 🫶

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🦋 ARGUMENTS 🦋

• nr — the bit to clear. the actual one. the chosen one. 🎯
• addr — the address to count from. it's giving "start here and go find it" energy. 📍

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💀 DESCRIPTION 💀

okay so this is the part where i need you to listen, because it's lowkey important:

this operation is non-atomic and can be reordered. no cap. it does not come with a seatbelt. 🚨

and here's the spicy bit — if two examples of this operation race, one can appear to succeed but actually fail. appear. it's fake success. it's the "i sent the text" of kernel operations, ngl. 😭

so if you're doing multiple accesses, you must protect them with a lock. periodt. not optional, not a suggestion, not a "we'll fix it later" — a lock. 🔒

bestie please. i am begging you. use a lock. i don't want to see your race condition on the timeline. 🙈

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🧚 SEE ALSO 🧚

• kernel file: ./tools/include/asm-generic/bitops/non-atomic.h — like, the lore document, go read it 📖
• ___set_bit(9)
• ___change_bit(9)
• ___test_and_set_bit(9)
• _test_bit(9)

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🌈 the cute sign-off 🌈

this lil beauty came from ./tools/include/asm-generic/bi... (yes, it's abbreviated, that's just how the page do) and it's dated October 2026. so it's from the future, kind of. main character energy, honestly. ⭐👑

slay, go clear your bits, and remember the lock. 💖✨