The `modify` method executes a read-modify-write operation on the `cr1` control register, as can be verified with the following disassembly code. Notably, the `read` variable is assigned a constant `true` within the example, which results in the elimination of the if comparison due to compiler optimization.

```
 8004a5e:	f645 4400 	movw	r4, #23552	@ 0x5c00
 8004a62:	4605      	mov	r5, r0
 8004a64:	f2c4 0400 	movt	r4, #16384	@ 0x4000    // r4 holds cr1 address
 8004a68:	4688      	mov	r8, r1
 8004a6a:	6820      	ldr	r0, [r4, #0]              // read cr1 [first read]
 8004a6c:	466e      	mov	r6, sp
 8004a6e:	f440 7080 	orr.w	r0, r0, stm32-rs#256	@ 0x100   // modify start bit
 8004a72:	6020      	str	r0, [r4, #0]              // write cr1 [first write]
 8004a74:	6820      	ldr	r0, [r4, #0]              // read cr1 [second read]
        /*  Code breaks if IRQ here  */
 8004a76:	f440 6080 	orr.w	r0, r0, #1024	@ 0x400   // modify ack bit
        /*  Code breaks if IRQ here  */
 8004a7a:	6020      	str	r0, [r4, #0]              // write cr1 [second write]
```

The issue arises when an IRQ occurs between the second read and write operations. Due to the CPU's faster execution speed compared to the peripheral, it is highly likely that the second read operation will still observe the `cr1` register with the `start` bit set. However, if an IRQ is serviced, the peripheral may have sufficient time to complete generating the start condition, resulting in the hardware clearing the `start` bit in the peripheral's `cr1` register. Nonetheless, the copy of the `cr1` register stored in `r0` during the second read operation will still retain the `start` bit set. Consequently, when the second write operation is executed on `cr1`, the `start` bit is once again set. As a result, the peripheral attempts to generate a second start condition, leading to a situation where the I²C peripheral becomes unresponsive in my specific case.

The solution is to swap the two method calls. Once the `ack` bit is set, it will not be automatically cleared, thus we are safe even if being interrupted in between.